1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus, a method for driving the photoelectric conversion apparatus, and an information processing apparatus having the photoelectric conversion apparatus and, more particularly, to a photoelectric conversion apparatus suitably used in X-ray image pickup apparatus, facsimile devices, scanners, and so on, a driving method of the photoelectric conversion apparatus, and an information processing apparatus provided therewith.
2. Related Background Art
FIG. 1 is a schematic circuit diagram to show the schematic structure of an example of the photoelectric conversion apparatus. In the figure, each pixel is composed of a photoelectric conversion element (a photodiode P1 to P4 in this example) and a thin film transistor (hereinafter abbreviated as TFT) T1 to T4. Numeral 1 denotes a power source connected to the photoelectric conversion elements, for applying the bias voltage thereto.
Charges generated in the respective photoelectric conversion elements P1 to P4 by incident light are transferred to a reading unit 2 by the thin film transistors (hereinafter called TFTs). The reading unit 2 is normally composed of amplifiers, an analog multiplexer, an A-D converter, a memory, etc. not illustrated. Further, numeral 3 designates a gate drive unit for applying a gate pulse Vg1 or Vg2 for control of on/off of the TFTs to the gate electrodes of the TFTs T1 to T4. The gate drive unit 3 is normally comprised of a shift register (not illustrated) or the like.
The photoelectric conversion elements P1 to P4 and the TFTs T1 to T4 are normally made of amorphous silicon materials or the like.
FIG. 2 is a timing chart to explain an example of reading operation of the photoelectric conversion apparatus. In the figure xe2x80x9cLightxe2x80x9d represents the timing of irradiation of light. After photocharges are accumulated in the respective photoelectric conversion elements P1 to P4 by the light irradiation, the gate drive unit 3 applies the gate pulse, as indicated by Vg1 and Vg2, to switch the TFTs T1, T3 on and then switch the TFTs T2, T4 on, whereby the charges generated by the light are transferred to the reading unit 2. The transferred charges are amplified, undergo A-D conversion, and are stored as image signals in the memory in the reading unit 2, and the signals are outputted to a monitor or the like as occasion may demand.
It is, however, commonly known that the performance of TFTs is degraded, that is, the threshold voltage Vth varies during the operation, in cases of TFTs made of the amorphous silicon materials. Particularly, where the photoelectric conversion apparatus is composed of an array of many pixels, variations etc. in production can cause variations in degrees of degradation of the TFTs. There are cases wherein some heavily degraded TFTs fail to transfer the charge successfully, so as to lower the output of pixels, compose defective pixels, and degrade the image quality.
In order to correct the variations of output, a potential method employed was to detect the defective pixels caused by the operation, based on a white image obtained under irradiation of light or X-rays or the like. It is, however, difficult to irradiate a large area with uniform light in general, and there were some cases wherein normal pixels were detected as defective pixels because of dust or the like on an illumination system or on the apparatus.
As described above, the photoelectric conversion apparatus had the problem of degradation of image quality, where the defective pixels appeared due to the degradation or the like of the TFTs during the operation. Further, the apparatus had another problem that it was considerably hard to accurately detect the defective pixels appearing during the operation per se.
The present invention has been accomplished in view of the above problems and an object of the present invention is to provide a photoelectric conversion apparatus, a driving method thereof, and an information processing apparatus provided therewith which permit accurate detection of the defective pixel or the like appearing during the operation of the photoelectric conversion apparatus or due to secular change of TFTs and which permit compensation for the defective pixels, so as to obtain a good image without substantial degradation of image quality.
Another object of the present invention is to provide a photoelectric conversion apparatus for reading information by arraying a plurality of pixels, each comprising a photoelectric conversion element and a thin film transistor connected to the element, and applying a voltage to gate electrodes of the thin film transistors to turn the thin film transistors on, the photoelectric conversion apparatus comprising a controllable power source for electrically charging the photoelectric conversion elements by changing a voltage applied to electrodes of the photoelectric conversion elements to which the thin film transistors are not connected, from a first voltage applied during normal reading to a second voltage and applying the second voltage to the electrodes in a dark state, and comparing means for comparing outputs read out of the charged photoelectric conversion elements with a predetermined threshold value to detect a defective pixel, and also to provide an information processing apparatus having the photoelectric conversion apparatus.
A further object of the present invention is to provide a method for driving a photoelectric conversion apparatus for reading information by arraying a plurality of pixels, each comprising a photoelectric conversion element and a thin film transistor connected to an output of the element, and applying a voltage to gate electrodes of the thin film transistors to turn the thin film transistors on, the apparatus having a reading mode and a self-diagnosis mode, the driving method comprising steps of electrically charging the photoelectric conversion elements by changing a voltage applied to electrodes of the photoelectric conversion elements to which the thin film transistors are not connected, from a first voltage applied in the reading mode to a second voltage and applying the second voltage to the electrodes in a dark state in the self-diagnosis mode, and comparing outputs read out of the charged photoelectric conversion elements with a predetermined threshold value to detect a defective pixel.
The present invention described above achieves the following operation; in the self-diagnosis mode the controllable power source changes and applies the voltage applied to the photoelectric conversion elements in the dark state, thereby charging the photoelectric conversion elements, not optically, but electrically, the charges are read out by the reading means, and the read outputs are compared with the predetermined threshold by the comparing means, so as to permit detection of the defective pixel.
Since the means for detecting the defective pixel by self-diagnosis has the function of switching two activity states of the reading mode and the self-diagnosis mode, the self-diagnosis can be performed even after activation of the apparatus by switching the mode into the self-diagnosis mode to find a defect due to a degraded TFT during the normal reading operation. Namely, the self-diagnosis can be performed at will of user or serviceman upon on of the main power supply, or by switching a changing switch.
By storing positional information of each defective pixel detected in the memory, the position of the defective pixel can be identified accurately and compensation by compensation means becomes easier.